1. Field of the Invention
This invention relates to an instrument for measuring the length of an intraocular length, such as an axis of a subject's eye by projecting a measuring beam of light onto the eye.
2. Description of the Prior Art
A conventional instrument in the art is known in which a position of the cornea of the eye with respect to the instrument is obtained by the aid of geometrical optics, a position of the eye fundus with respect thereto is obtained by the aid of physical optics, and an eye axis length is measured from a difference between the two positions without contact of the instrument with the eye.
This conventional instrument first projects a measuring beam of linearly polarized light emitted from a light source, such as a laser diode, onto the eye fundus and a reference surface. Interference is then made between light beams reflected from the eye fundus and the reference surface. After that, the resultant interference light is received by a light receiving element and causes interference fringes to occur on the light receiving surface of the element. An output signal from the element is then processed by an arithmetic circuit to determine the position of the eye fundus with respect to the instrument.
If noises, such as those due to abnormal refraction, do not occur in an optical path between the instrument and the eye fundus, the two reflected light rays can be caused to interfere with each other without giving any optical treatment to the rays. The position of the eye fundus determined as described above is thus satisfactory in accuracy.
However, there are some cases where the eye ball of the subject's eye has abnormal refraction (birefringence). In that case. linearly polarized light projected onto the eye becomes two rays of polarized light, i.e., an ordinary ray and an extraordinary ray, when the projected light is reflected from the eye fundus. These two rays have a phase difference.
The intensity of the interference light on the light receiving surface in a case where interference is made between the light rays reflected from the eye fundus and the reference surface having a phase difference is far less than that in a case where interference is made between the light rays having no phase difference. (The light ray reflected from the eye fundus is a measuring light ray.) That is, if light rays reflected from the eye fundus and the reference surface having a phase difference are caused to interfere with each other, an output signal from the light receiving element becomes unclear, in other words, the contrast between interference fringes produced on the light receiving surface decreases. Therefore, measurement of the eye axis length is inaccurate.